I had a 1978 GMC ½ ton 4X4 with the full time hubs and transfer case.
I never removed the solid hubs, or installed the part time kit in the transfer case, but I did rebuild the rear axle because it leaked from everywhere.
I ran a 305 with a new cam and lifters slightly over stock specifications, the TH-350 transmission got rebuilt as it lost first and reverse along with leaking from every place it could, I had a set of 31X10.5X15 tires and the gears in it were 3.55, I got 18 mpg when I did more highway driving.
I had a 77 Chevy that came form the factory with a 400, TH-350, NP-203, and after replacing the engine with a 350 (the 400 spun a rod bearing) and swapping a set of 33X12.5X15 tires on it, I got a rough average of 13 mpg, and this truck had the part time kit and hubs BUT had a set of 3.08 gears.
Over the years I ended up using the engine out of the first above mentioned truck, the transmission out of the second above mentioned truck and a NP-205 transfer case along with a 1983 8.5 inch GM front axle and 1974 10.5 inch rear axle containing 4.10 gears, along with the 33X12.5X15 tires from the second above mentioned truck and got an average of 14 mpg.
Your mileage will vary greatly with the amount of highway use, stop & go traffic, idle time, etc. but the tires need to match the gearing you have to keep the engine rpm's in the lower end of the power band to get better mileage, a general rule of thumb for mileage and power trade-off is keeping the engine revving around 2100 rpm's.